US7200123B2 - Method and apparatus for the power control system outer loop of a mobile communications system - Google Patents
Method and apparatus for the power control system outer loop of a mobile communications system Download PDFInfo
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- US7200123B2 US7200123B2 US10/538,159 US53815905A US7200123B2 US 7200123 B2 US7200123 B2 US 7200123B2 US 53815905 A US53815905 A US 53815905A US 7200123 B2 US7200123 B2 US 7200123B2
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- 238000000034 method Methods 0.000 title claims abstract description 58
- 238000010295 mobile communication Methods 0.000 title claims abstract description 13
- 230000001413 cellular effect Effects 0.000 claims abstract description 15
- 238000009826 distribution Methods 0.000 claims description 35
- 238000005562 fading Methods 0.000 claims description 28
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W52/00—Power management, e.g. TPC [Transmission Power Control], power saving or power classes
- H04W52/04—TPC
- H04W52/18—TPC being performed according to specific parameters
- H04W52/22—TPC being performed according to specific parameters taking into account previous information or commands
- H04W52/225—Calculation of statistics, e.g. average, variance
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W52/00—Power management, e.g. TPC [Transmission Power Control], power saving or power classes
- H04W52/04—TPC
- H04W52/06—TPC algorithms
- H04W52/12—Outer and inner loops
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W52/00—Power management, e.g. TPC [Transmission Power Control], power saving or power classes
- H04W52/04—TPC
- H04W52/18—TPC being performed according to specific parameters
- H04W52/24—TPC being performed according to specific parameters using SIR [Signal to Interference Ratio] or other wireless path parameters
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02D—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN INFORMATION AND COMMUNICATION TECHNOLOGIES [ICT], I.E. INFORMATION AND COMMUNICATION TECHNOLOGIES AIMING AT THE REDUCTION OF THEIR OWN ENERGY USE
- Y02D30/00—Reducing energy consumption in communication networks
- Y02D30/70—Reducing energy consumption in communication networks in wireless communication networks
Definitions
- This invention relates to the field of wireless communications and, more specifically, to the outer loop of the power control system of a cellular telephony network.
- ETSI European Telecommunications Standards Institute
- UMTS Universal Mobile Telecommunications System
- WCDMA Wideband Code Division Multiple Access
- 3G Third-generation
- Such a power control system is described below in a general manner, as the functionality of the outer loop, for which a method is proposed in this invention, results from other system components.
- a power control system is needed in WCDMA-based cellular networks because this technology is limited by interference, as all users share the same frequency spectrum and their codes are not fully orthogonal (see Holma & Toskala: “WCDMA by UMTS, Radio Access for Third Generation Mobile Communications”, John Wiley & Sons.).
- the ultimate goal of the power control system in WCDMA systems is to attain the quality of service required in a specific link (uplink or downlink) with a minimum transmit power level. This invention is specifically centred on this aspect.
- a WCDMA power control system is implemented by three differentiated loops:
- Open loop during the random access process at the start of a connection, the base/mobile station estimates the power loss in the uplink/downlink and adjusts its transmit power accordingly.
- Closed or inner loop Also known as the fast power control (1500 Hz), comprised of the following three stages:
- the corresponding reception terminal compares the desired signal to interference received ratio (SIR rec ) and the desired signal to interference target ratio SIR tgt , which depends on the quality of service required for this specific link that is determined by the outer loop as described further below.
- the same reception terminal sends power control bits indicating whether the transmit power must be increased (if SIR rec ⁇ SIR tgt ) or reduced (if SIR rec >SIR tgt ) by a certain amount (normally 1 dB).
- the transmission unit increases or reduces its power by the previously determined amount.
- Outer loop it is much slower than the closed loop (10–100 Hz), establishing the desired signal to interference target ratio SIR tgt that allows maintaining a predetermined quality goal.
- One measure of the link quality is the Frame Error Rate (FER), which depends on the desired signal to interference ratio SIR.
- FER Frame Error Rate
- the inner loop helps to maintain the SIR near the SIR tgt , the FER is ultimately determined by this target value.
- the SIR tgt value must be adjusted to a suitable value for this environment.
- the patent application titled “Symbol Error Based Power Control For Mobile Telecommunications System” (Carl Weaver, Wei Peng), Ser. No. 08/346800, of 30 Nov. 1994, describes a method based on the Symbol Error (SE) that improves the loop performance in dynamic fading environments.
- SE Symbol Error
- This procedure is based on the premise that the Symbol Error Rate (SER) and the FER are highly correlated, and thus attempts to maintain the SER close to a predetermined target SER value. As before, this is achieved by increasing or decreasing the SIR tgt .
- the method and apparatus disclosed for the power control system outer loop of a mobile communications system allow determining the fading margin for the desired signal to interference ratio (M (SIR) (dB)), and thus the SIR tgt for a quality of service criterion given by the outage probability (P outage ) and statistical moments characteristic of the radioelectric channel being considered.
- M desired signal to interference ratio
- P outage outage probability
- This allows complying with the aforementioned QoS with the minimum power level needed which, as this technology is limited by interference, also allows the optimization of the system capacity.
- the quality criteria on which this invention is based on instead of the FER as in the previous cases, is the outage probability parameter, which is another quality parameter often used in cellular infrastructures.
- the P outage value is often determined in the cellular network planning stage, and among other parameters depends on:
- This invention solves the process inverse of that described in the previous paragraph, to apply it for obtaining the desired signal to interference target ratio SIR tgt of the outer loop in a WCDMA power control system.
- an iterative method is applied that allows obtaining the margin above the SIR median required to comply with the outage probability specification (QoS) for this specific link, as well as for second-order statistical moment values (such as the typical deviation) which are dynamically estimated and thus are compatible with the various fading conditions that characterise an environment at a given time.
- the method described is based on a stochastic propagation method and therefore does not intend to estimate the value of the desired signal to interference ratio (SIR) required to comply with the Quality of Service (QoS), but instead intends to provide the value of the required margin (M (SIR) (dB)) above the median of its probability density function.
- SIR desired signal to interference ratio
- QoS Quality of Service
- M required margin
- this invention provides a mathematically rigorous method for maintaining the quality of service (QoS) of a specific link that can also respond to constant statistical variations of the radioelectric channel, unlike the previously cited methods based on FER measurements.
- QoS quality of service
- FIG. 1 is a block diagram of a portion of a mobile communications system related to the principles on which embodiments of the invention are based.
- FIG. 2 is a block diagram of a portion of a base station or mobile station related to embodiments of the invention.
- FIG. 3 is a flow diagram illustrating one embodiment of a method of the invention corresponding to the power control system outer loop of a WCDMA-based cellular network.
- FIG. 4 is a flow diagram illustrating one embodiment of a method of the invention at a higher level, corresponding to the power control system outer loop of a WCDMA-based cellular network.
- the present invention is applicable to macro or microcellular channels in rural, suburban and urban environments.
- the fluctuations of the field values are considered along points equidistant from the transmitter (variations with locations) which follow a lognormal law and time variations in the same point, resulting from multipath propagation, described by a Rayleigh distribution.
- variation with locations is referred to as slow fading or shadowing and multipath propagation variation is referred to as fast or Rayleigh fading.
- S and I are respectively the power values of the desired and interfering signals.
- F S ⁇ ( S 0 ) ⁇ - ⁇ S 0 ⁇ f S ⁇ ( S 0 ) ⁇ d S ( 1 )
- F S is the aforementioned distribution function for the random variable S, which corresponds to the desired signal.
- this will be a Nakagami-Rice-Lognormal or Rayleigh-Lognormal distribution, depending on whether or not there is a direct beam between the emitter and the receiver.
- the integral of equation (1) cannot be expressed as elementary functions in either case, and must be calculated by numerical methods.
- margin in dB is defined by the following expression:
- the desired signal comprises one deterministic component and several random components.
- the resulting probability density function is the Nakagami-Rice function:
- the Rice factor is the parameter estimated for a dynamic characterisation of the channel, as will be seen later.
- the mean quadratic value of the random component can be expressed as a function of the Rice factor as follows:
- the random variable describing the previous situation (Nakagami-Rice) is multiplied by a random variable that follows a Lognormal distribution.
- dB the above is equivalent to adding a random component following a Gaussian or Normal distribution characterised by a typical deviation • N , so that the resulting probability density function will be the convolution of the previous Nakagami-Rice function with this Gaussian function.
- F NR - LN ⁇ ( m 0 ) ⁇ - ⁇ ⁇ ⁇ F NR ⁇ ( m 0 - u ) ⁇ f N ⁇ ( u ) ⁇ ⁇ d u ( 4 )
- F NR according to equation (3) will be given by:
- the object is to obtain an expression using numerical integration methods to then introduce it in the previous expression.
- Bessel function we first approximate the Bessel function with the following form:
- I 0 ⁇ ( ⁇ ) 1 2 ⁇ ⁇ ⁇ ⁇ 0 2 ⁇ ⁇ ⁇ e ⁇ ⁇ ⁇ cos ⁇ ⁇ ⁇ ⁇ d ⁇
- a suitable change of variable gives an integral with the Gauss-Hermite weight function (e ⁇ x2 ) and integration interval ( ⁇ , ⁇ ) is left, so that a formula of this type is applied to obtain the desired result.
- the objective is to solve the integral of equation (2) for the two distribution functions of the desired signal: Rayleigh-Lognormal (F R ⁇ LN ) and Nakagami-Rice-LogNormal (F NR ⁇ LN ).
- the random variable modelling the lognormal component of the desired signal is transformed into a Gaussian distribution (with typical deviation • N ) and that describing the statistical distribution of the interfering signals is also a Gaussian distribution with typical deviation ⁇ I .
- the desired and interfering signals expressed in dB are linearly related in the SIR calculation (subtracted) and the random variable resulting from a linear combination of two statistically independent Gaussian distributions (as is the case here) is also Gaussian, with a variance that is the sum of the individual variances according to the convolution theorem (see Peyton Z. Peebles, JR.: “Probability, Random Variables, and Random signal principles”, McGRAW-HILL INTERNATIONAL EDITIONS Electrical Engineering Series, 1987).
- the object is to obtain the M (S/I) (dB) margin, which for certain statistical conditions will fulfil a given outage probability specification P Outage (QoS).
- P Outage F SIR
- P Outage F SIR
- the object is to obtain the zeroes of the function g. To do so, from an initial value for the margin M (S/I)0 ⁇ 0 we will apply the expression:
- M (S/I) ⁇ i+1 is calculated as a function of M (S/I) ⁇ i •
- the process is ended when the difference between the two last values found for the margin differ by less than a given value, which in this case is set at 10 ⁇ 4 .
- the margin values determined by the above method must be corrected by the corresponding factor if diversity techniques or RAKE receptors are used (see Don Torrieri: “Instantaneous and Local-Mean Power Control for Direct-Sequence CDMA Cellular Networks”, U.S. Army Research Laboratory).
- the base station 102 includes processors, memories, interface cards and embedded software programs.
- the figure contains an RNC (Remote Network Controller) 101 which among other functions processes calls; two base stations: 102 , 103 ; and one mobile station 104 represented by the vehicle icon.
- the two base stations and the mobile station are representative of end points of the wireless interface.
- Each base station shall be associated to one RNC 101 through the land lines 105 and 108 .
- the mobile station 104 communicates with the base station 102 through the downlink signal 107 and the uplink signal 108 .
- FIG. 2 shows the part of both stations ( 102 and 104 ) that includes the principles on which this invention is based.
- Both the base station and mobile station comprise a controller 201 , an emitter 202 and a receiver 203 .
- the received signal corresponds to the uplink 108
- the received signal is the downlink signal 107 , both arriving at the controller 201 through the receiver 203 .
- the power control system will deliver through the emitter 202 a command instructing the other station to increase or decrease its power depending on the result of the optimisation method described below, which sets the target desired signal to interference ratio that acts as the threshold in the closed loop of this power control system.
- One of the components of this power control system is the outer loop, for which this invention proposes a new method, as mentioned above.
- the stages that take place in the controller 201 and that correspond to this outer loop are shown in detail in FIG. 3 .
- both the base 102 and the mobile station 104 estimate the desired signal to interference ratio received SIR rec (see Sáez Ruiz, Juan Carlos: “ Una Arquitectura Hardware para la Estima Terms de la Rela Agriculture Se ⁇ al a Interferencia ensselas WCDMA” (A Hardware Architecture for Estimation of the Signal to Interference Ratio in WCDMA Systems), Department of Electroscience, Digital ASIC University of Luna), as well as the corresponding statistical parameters for each type of communication (see Ali Abdi, Georgios B.
- the aforementioned Newton-Raphson iteration method is used to obtain the dB margin required to fulfil the aforementioned QoS.
- the previous margin is used to calculate the new desired signal to interference target ratio SIR tgt , which shall be the reference threshold for the closed loop of the power control system.
- N typical deviation of the lognormal fading of the desired signal (in dB);
- ⁇ I typical deviation of the Gaussian random variable that describes the variation of the interfering signals (in dB);
- T n outer loop period during which the channel statistical characteristics are estimated (in secs.);
- P Outage desired outage probability, in this case it defines the Quality of Service (QoS) of the link
- SIR max maximum allowed SIR value for the link being considered (in dB);
- the signal to interference ratio SIR is estimated in the block 301 , where the corresponding hardware architecture is contained.
- the second order statistical moments are estimated during the outer loop period T n in the block 302 , always taking as input the SIR provided by block 301 ; thus, for the NLOS case • N and ⁇ I are estimated, and in the direct beam case (LOS) the corresponding Rice factor is also estimated.
- the block 303 is the one most representative of the present invention, as in it the Newton-Raphson iteration method is applied which allows obtaining the margin (in dB) that fulfils the outage probability specification P Outage for the statistical characteristics estimated in 302 .
- the function g(M (S/I) ⁇ 0 ) is calculated according to (9) and applying (10) new values are obtained for the margin until the difference between the last two values, M (S/I) ⁇ 0 and M (S/I) ⁇ i+1 , is less than 10 ⁇ 4 .
- the target SIR corresponding to this margin which we call SIR tgt ⁇ n is calculated in block 304 .
- the following conditions are established to prevent exceeding the allowed SIR limits:
- FIG. 4 shows a representation at a higher level of the method of the invention of FIG. 3 .
- the block 401 is equivalent to the block 301 of the latter figure
- block 402 represents the functionality described in 302
- 403 (equivalent to 303 ) includes the Newton-Raphson iteration method and is known as the channel Quality Controller.
- the maximum and minimum power limitations have been collected in block 404 of the aforementioned figure.
- the invention can be applied to terrestrial, marine or aeronautical satellite-based systems (geostationary or otherwise) (see G. E. Corazza, F. Vatalaro, “A Rice-Lognormal Terrestrial and Satellite Channel Model”, IEEE Trans. Veh. Technol., vol. 43, no. 3, p. 738–742, 1994). These models also reproduce the indoor propagation conditions (see Tadeusz A. Wysocki, Hans Jürgen Zepernick, “Characterization of the indoor radio propagation channel at 2.4 GHz”, 3–4 2000, Journal of Telecommunications and information Technology), so that the invention may be used for the design and power control of systems used in this environment.
- the invention can be applied to standards other than WCDMA, as well as to control the power of any signal received by the base or mobile stations.
- the invention can be used as a link level simulator by the operators in the cellular planning stage (see Moreno González J. A., Miranda Sierra J. L., Eliseo Barandilla Torregrosa I., Lorca Hernando J., “ Simulador de enlaces para el ista UNTS en modo FDD” (Links Simulator for the UMTS system in FDD mode), Telefónica Móviles Espa ⁇ a, Telefónica I+D).
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Abstract
Description
S/I(dB)=S(dBm)−I(dBm)
Note: the measurement in dBm is obtained by applying the 10 log(x) function to a measurement in mW.
M (S/I)(dB)=−S/I(dB)
ƒS/I(M (S/I))=∫∞ ∞ƒ S(M S/I −u)ƒI(u)du
and for the distribution function, which provides the outage probability for a given margin M(S/I)0, we have:
where FS is the aforementioned distribution function for the random variable S, which corresponds to the desired signal. As mentioned above, this will be a Nakagami-Rice-Lognormal or Rayleigh-Lognormal distribution, depending on whether or not there is a direct beam between the emitter and the receiver. The integral of equation (1) cannot be expressed as elementary functions in either case, and must be calculated by numerical methods.
where •I is the typical deviation of the normal distribution in dB, which as will be seen further below is one of the dynamically estimated parameters used to evaluate the statistical variations of the channel.
Numerical Approximations for the Desired Signal Distributions:
Rayleigh-Lognormal (Suzuki Model)
where
In natural units this margin is:
To thereby obtain:
Y=1n(x)
where:
- 2b: the mean quadratic value of the random component.
- C: the effective value of the deterministic component
- Io: Modified Bessel function of the first kind, zero order.
2b+c 2=1
FNR according to equation (3) will be given by:
σN−I(dB)=√{square root over (σN 2+σI 2)}
calculated as:
P Outage =F SIR|NLOS [M SIR
P Outage =F SIR|LOS └M SIR
F└M SIR(dB),
where mi represents the second order statistical moments corresponding to each case. As the values of these statistical moments are constant during the iteration on which the method is based, from now on the margin M(S/I) shall be considered to be the only variable of F.
g└M SIR(dB),
g′[M (S/I)(dB)i ]=F′[M (S/I)(dB)]
σN−1(dB)=√{square root over (σN 2+σ1 2)},
-
- M(S/I)=20 dB
- •N=4 dB
- σI=4 dB
-
- POutage=0.0223
Inverse Problem for NLOS:
- POutage=0.0223
-
- POutage=0.0223
- •N=4 dB
- σI=4 dB
-
- M(S/I)=20.0004 dB
Direct Problem for LOS:
- M(S/I)=20.0004 dB
-
- M(S/I)=20 dB
- •N−I=4 dB
- K=0 dB
-
- POutage=0.0113
Inverse Problem for LOS:
- POutage=0.0113
-
- POutage=0.0113
- •N−I=4 dB
- K=0 dB
-
- M(S/I)=19.9891 dB
Claims (9)
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ES200202947A ES2214121B1 (en) | 2002-12-20 | 2002-12-20 | METHOD AND APPARATUS FOR THE EXTERNAL LOOP OF THE POWER CONTROL SYSTEM OF A MOBILE COMMUNICATIONS SYSTEM. |
PCT/ES2003/000630 WO2004057773A1 (en) | 2002-12-20 | 2003-12-12 | Method and apparatus for the outer loop of the power control system of a mobile communication system |
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US20060211377A1 (en) * | 2004-09-14 | 2006-09-21 | Shoemake Matthew B | Detection and mitigation of interference and jammers in an OFDM system |
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Also Published As
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ES2214121B1 (en) | 2006-01-01 |
CA2494242C (en) | 2010-04-13 |
WO2004057773A1 (en) | 2004-07-08 |
KR100804331B1 (en) | 2008-02-15 |
ATE396554T1 (en) | 2008-06-15 |
DE60321224D1 (en) | 2008-07-03 |
MXPA05000699A (en) | 2005-08-16 |
JP2006511131A (en) | 2006-03-30 |
JP4373924B2 (en) | 2009-11-25 |
CN100479343C (en) | 2009-04-15 |
BR0315985A (en) | 2005-10-04 |
CN1692569A (en) | 2005-11-02 |
EP1575185A1 (en) | 2005-09-14 |
ES2214121A1 (en) | 2004-09-01 |
KR20050083629A (en) | 2005-08-26 |
EP1575185B1 (en) | 2008-05-21 |
RU2309541C2 (en) | 2007-10-27 |
RU2005122953A (en) | 2006-01-20 |
US20060166691A1 (en) | 2006-07-27 |
AU2003288280A1 (en) | 2004-07-14 |
CA2494242A1 (en) | 2004-07-08 |
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